The present invention is concerned with a catalytic composition and a process which uses that composition for the dimerization, codimerization and oligomerization of olefins, and, in particular, propylene, the composition resulting from dissolving a nickel compound mixed or complexed with a phosphine in the liquid mixture of ionic type of quaternary ammonium halide and/or quaternary phosphonium halide, aluminium halide, an aromatic hydrocarbon and optionally an aluminium alkyl compound.
French Patent 2611700 describes the use of liquids of the ionic type formed from quaternary aluminium halides and ammonium halides as solvents of organometallic nickel complexes for dimerization catalysis of olefins. The use of such media which are immiscible with aliphatic hydrocarbons, particularly with end products of olefin dimerization makes better use of homogeneous catalysts possible. U.S. Pat. No. 5,104,840 describes a liquid composition of the ionic type resulting from contacting quaternary ammonium halides and/or quaternary phosphonium halides with aluminium alkyl dihalides and possibly also an aluminium trihalide. This same patent describes the use of these media as solvents of transition metal complexes, particularly nickel complexes not containing a nickel-carbon bond which are transformed into olefin oligomerisation catalysts. Hereinafter, these media will be called "molten salts" because they are in a liquid state at a moderate temperature.
During the work undertaken, it has been seen that the most active and most stable nickel catalysts are obtained in "molten salts" constituted by a molar equivalent of ammonium halide and/or phosphonium halide with one equivalent and more of aluminium trihalide, and optionally any quantity of aluminium alkyl dihalide. This formulation was seen to be particularly worthwhile because the nickel complexes dissolved there had a high degree of catalytic activity which was constant with the passage of time.
However, it has been seen that under such conditions "the phosphine effect" described by G. Wilke et al in Ind. Eng Chem., 1970, 62, No. 12, P34 and in GB Patent 1.058.680 which manifests itself by the influence which the substituents provided by the phosphorus atom have on the way in which the propylene molecules are interlinked during catalytic dimerization by nickel, rapidly disappears with the passage of time. This unexplained phenomenon has unfortunate consequences since it is not possible to obtain the desired selectivities.